Short circuit and over-current conditions can result in failure of power devices such as IGBTs (insulated gate bipolar transistors) and other high power semiconductor switching devices (e.g., MOSFETs, etc.) if appropriate remedial action is not taken within a short period of time, generally on the order of a few microseconds. The IGBT short circuit internal failure mechanism is different than the failure mechanism in hard switching inductive turn-off failures. A short circuit condition and the consequent high current through the device results in local heating close to the gate oxide in the IGBT and can severely degrade the device. Excessive power dissipation during the fault leads to chip heating, which can eventually destroy the device. Intelligent power modules and advanced gate driver chips have been developed for protection of IGBTs. However, there are no benchmarks for the performance of these circuits.
Several approaches to the protection of IGBTs have been proposed and studied. These techniques include the use of a capacitor to reduce the gate voltage after the fault. R. Chokawala, et al. "IGBT Fault Current Limiting Circuit," IEEE-IAS Conf. Rec., 1993, pp. 1339-1345. This approach has the limitation that the IGBT current may be shut-off and then turned back on again depending on the initial charge condition of the capacitor and the value of its capacitance. In addition, the capacitor must have a relatively large capacitance value to prevent the capacitor voltage from drifting back to the normal on-state gate voltage. Multiple stages of clamping have also been proposed to increase the endurance time and to reduce the turn-off current level. A pure zener based clamp can be used but has the drawback that the clamping gate voltage can be much larger than desired under the transient conditions of the fault. A protection circuit topology has been proposed wherein the zener and capacitive method is used to limit fault currents. R. Chokawala, et al., "Switching Voltage Transient Protection Schemes for High Current IGBT Modules," IEEE-APEC Conf. Rec. 1994, pp. 459-468. This type of circuit is effective in eventually clamping the fault current level, but it does not limit the large peak current that flows immediately after the fault due to the delay in the operation of the circuit.
In addition to the control of the gate voltage after detection of a fault to limit the current flowing through it, methods have been proposed to softly turn off the IGBT after the fault and to reduce the over-voltage that is due to the turn-off di/dt. See, e.g., H. G. Eckel, et al., "Optimization of Short Circuit Behavior of NPT IGBT by Gate Drive," EPE Conf. Rec., 1995, Vol. 2, pp. 213-218. The purpose of such circuits is to control the over-voltage on the device caused by the parasitic inductance of the power circuit while the device is turning off large currents.
Several problems remain unresolved in the active protection of IGBT modules from fault currents. One such problem is that the use of a large on-state gate voltage to reduce conduction losses through the device makes the fault situation more problematic and dangerous, because it leads to very high fault current, large, instantaneous power dissipation, and the possibility of latching in the device. Thus, a trade-off has been required between the conduction loss during normal operation of the IGBT and the short circuit current magnitude during a fault condition.
Another problem with prior approaches is that the precise detection of fault current levels is difficult if current sensors are not used in series with the IGBTs. In particular, in the case of a large fault inductance (a "soft fault") it is difficult to precisely recognize the over-current conditions using the desaturation technique, which is a common method for identifying a fault situation. This difficulty is due to the reduced voltage drop in the IGBT under low di/dt conditions, as well as the slow dynamics in the electronic components of the detection circuit.
A further complication is that the initial value of the short circuit current is the highest due to the increase gate voltage caused by the Miller capacitance. It is thus not easy to reduce the initial peak current because activation of the protection circuit should be prevented during the normal turn-off transient conditions of the IGBT as well as during the presence of noise phenomena caused by the normal switching of the IGBTs in a power converter circuit.
In addition to a fast, reliable and safe detection of short circuit conditions, after the short circuit condition has been detected and the IGBT is to be shutdown the falling rate of the IGBT current should be controlled to reduce the over-voltage stress on the device. The over-voltage level across the device can become much larger than the rated voltage if the large collector current is turned off without any treatment, particularly with highly inductive loads. In addition, such soft turn-off techniques should take into account possible changes in the operating modes of the protection circuit.